CVE-2024-21357 is a high-severity remote code execution vulnerability affecting Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The vulnerability arises from a type confusion flaw (CWE-843) in the Windows Pragmatic General Multicast (PGM) protocol implementation. Type confusion occurs when a program accesses a resource using an incompatible type, leading to unexpected behavior. In this case, an attacker can exploit this flaw remotely without requiring authentication or user interaction, by sending specially crafted network packets to a vulnerable system's PGM service. Successful exploitation could allow the attacker to execute arbitrary code with system-level privileges, compromising confidentiality, integrity, and availability of the affected system. The CVSS 3.1 base score is 8.1, indicating high severity, with the vector string AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H, meaning the attack is network-based but requires high attack complexity, no privileges, and no user interaction. Although no known exploits are currently reported in the wild, the vulnerability poses a significant risk due to its potential impact and the critical nature of remote code execution flaws. The lack of available patches at the time of publication increases exposure for affected systems. This vulnerability specifically targets Windows 10 Version 1809, which is an older but still in-use version in some environments, particularly in legacy or industrial systems. The PGM protocol is used for reliable multicast communication, which is less common in typical enterprise environments but may be present in specialized network configurations or legacy applications.

For European organizations, the impact of CVE-2024-21357 could be substantial, especially for those still operating Windows 10 Version 1809 in production environments. Successful exploitation could lead to full system compromise, enabling attackers to steal sensitive data, disrupt operations, or deploy ransomware and other malware. Critical infrastructure sectors such as manufacturing, energy, and transportation that rely on legacy Windows 10 systems and multicast networking could be particularly vulnerable. The confidentiality, integrity, and availability of affected systems would be at high risk, potentially causing operational downtime and financial losses. Additionally, organizations in regulated industries (e.g., finance, healthcare) could face compliance violations if exploited. The high attack complexity somewhat reduces the likelihood of widespread exploitation, but targeted attacks against high-value assets remain a concern. The absence of known exploits in the wild currently provides a window for mitigation before active exploitation emerges.

European organizations should prioritize identifying and inventorying all systems running Windows 10 Version 1809, especially those using multicast networking features like PGM. Given the lack of an official patch at the time of this report, organizations should implement network-level mitigations such as blocking or filtering PGM traffic at firewalls and network segmentation to limit exposure. Disabling the PGM protocol or related services on systems where it is not required can reduce the attack surface. Monitoring network traffic for anomalous multicast packets may help detect exploitation attempts. Organizations should also plan to upgrade affected systems to a supported Windows version with security updates. Applying security best practices such as least privilege, endpoint detection and response (EDR) solutions, and regular vulnerability scanning will further reduce risk. Close coordination with Microsoft for patch availability and applying updates promptly once released is critical. Incident response teams should prepare for potential exploitation scenarios given the high impact of this vulnerability.